1. Field of the Invention
The present invention relates to the field of image-rejection mixers.
2. Prior Art
When an RF (radio frequency) signal comprising a plurality of channels of information on different frequencies is down shifted to downshift the desired channel to a predetermined IF (intermediate frequency) frequency (actually a frequency range), one or more other channels may be downshifted to fall into the same IF frequency range. In particular, consider an RF frequency band being downshifted by mixing with another frequency fLO. The mixing will provide sum and difference frequencies. For downshifting, the sum frequencies will be well above any frequency range of interest and easily filtered out of the resulting signal. For the difference frequencies however, when one or more channels in the RF signal are downshifted to the IF frequency range fIF, one or more other channels in the RF signal that are 2fIF away from the channels downshifted to the IF frequency range will appear in the −IF frequency range. Actually either of these IF frequency band signals could be the desired signal, depending on the application, with the other being commonly referred to as the image frequency or frequencies.
Once the image frequencies are combined with the desired signal, they cannot be separated, or the image frequencies eliminated. One approach for avoiding this is to bandpass filter the RF signal to eliminate the image frequencies from the RF signal prior to the mixer. This is difficult however, because of the sharp and variable frequency filters required. Another approach is to use what are referred to a image-rejection mixers, which by their design, will pass the desired downshifted signals and automatically eliminate the image frequencies from the image-rejection mixer output.
Two classic architectures for image-rejection mixing are shown in FIG. 1 and FIG. 2 (“CMOS Mixers and Polyphase Filters for Large Image Rejection”, F. Behbahani et al., IEEE Journal of Solid-State Circuits, Vol. 36, No. 6, June 2001). These mixers are composed of two mixing cells MC1 AND MC2, two 90° phase shifters and a local oscillator signal LO. In FIG. 1, the local oscillator signal LO is applied to both mixers MC1 AND MC2, but shifted 90° with respect to one of the two mixers. In FIG. 2, the local oscillator signal LO is applied to both mixers MC1 AND MC2 without phase shift, but the RF signal is shifted 90° with respect to one of the two mixers. By proper selection of the direction of the phase shifts, the components of the IF OUTPUT in the two signal paths through the image-rejection mixer in the desired frequency band will add to create the IF OUTPUT, whereas the components of the IF OUTPUT in the two signal paths through the image-rejection mixer in the image frequency band will subtract, and therefore be eliminated from the IF OUTPUT.
Because image-rejection mixers must sometimes operate over relatively wide frequency ranges, the 90° phase shifters are commonly implemented in semiconductor integrated circuits using polyphase networks (“Single-Sideband Modulation Using Sequence Asymmetric Polyphase Network”, M. J. Gingell, Electrical Communication, Vol. 48, No. 1 and 2, 1973). For high gain, the mixing cells are commonly implemented in such circuits using Gilbert-type mixers (U.S. Pat. No. 5,589,791).
Although both the polyphase networks and the Gilbert mixers have indisputable advantages as individual subcircuits, a direct implementation of an image-rejection mixer using these components has several drawbacks. An example is shown in FIG. 3, which shows a direct implementation of the diagram in FIG. 1. In FIG. 3, the RF stage is composed of npn transistors Q1-Q4 and degeneration resistors R. The inphase and quadrature local oscillators signals VLOI and VLOQ drive the mixers (mixing quads MQ1 and MQ2), and the output polyphase network has k sections consisting of resistors RPN1-RPNk and capacitors CPN1-CPNk. Only one of the output voltages vOUT1 and vOUT2 is used as the output of the image-rejection mixer. In FIG. 3, Q1, Q3, MQ1 and Q2, Q4, MQ2 are Gilbert-type mixers (mixing cells MC1 and MC2 in FIG. 1). First of all, if high linearity (high third-order input-referred intermodulation point, or IIP3) is desired for the image-rejection mixer, then degeneration must be used in the RF stage as shown, and that will limit the available headroom to the output stage (thus limiting the maximum achievable value for IIP3). The degeneration in the RF stage also reduces the conversion gain of the mixer. For bigger headroom and therefore better linearity (IIP3), the tail current source (4I0) may be absent, as the tail node may be connected directly to ground or to a tank circuit (whose resonant frequency is the RF frequency). More importantly, however, because the output polyphase network and the load resistors are in the DC current path of the output stage, the output headroom is severely reduced. especially in the case of low-voltage circuits with multiple-section polyphase networks, thereby reducing the linearity of the mixer. Increasing the output headroom is possible by reducing the load resistances and the resistances in the polyphase networks, but by doing this the conversion gain of the mixer is also reduced. Restoring the conversion gain is possible using additional gain stages at the output, with the drawback of having to use more supply current.
A possibility for avoiding the output headroom problem of the circuit in FIG. 3 is shown in FIG. 4, where the output polyphase network is taken out of the DC path of the quad collector currents and driven by two differential buffers BUF1 and BUF2. The drawback of this approach is that more supply current is needed for the buffers, especially for minimizing the linearity (IIP3) degradation that occurs at the buffer/polyphase network interface.
When up shifting from baseband to an RF frequency, rejection of the image frequencies is also desired. In the prior art, separate up-converters, polyphase filters and variable gain amplifiers have been used. Separate circuits for these functions consume more power and have a lower gain than the present invention.